In digital color image input apparatuses (such as digital scanners, digital still cameras, and the like), tristimulus color information (R, G, B) is obtained via a solid-state image sensing element (CCD) that serves as a color separation system. The tristimulus color information, which is obtained in a form of analog signals, is then converted to digital signals, which are used as input signals that represent input color image data (color information). Segmentation is carried out so that display or output is carried out most suitably according to the signals obtained via the image input apparatus. The segmentation partitions a read document image into regions of equivalent properties so that each region can be processed with image process most suitable thereto. This makes it possible to reproduce a good-quality image.
In general, the segmentation of a document image includes discriminating a text region, a halftone region (halftone area) and photo region (in another words, contone region, which is sometimes expressed as other region) in the document image to read, so that quality improvement process can be switched over for the determined respective regions. This attains higher reproduction quality of the image.
Furthermore, the halftone regions (image) have halftone varied from low frequencies to high frequencies, such as 65 line/inch, 85 line/inch, 100 line/inch, 120 line/inch, 133 line/inch, 150 line/inch, 175 line/inch, 200 line/inch, and the like. Therefore, various methods have been proposed for determining halftone frequencies so as to perform suitable process according to the determination.
For example, Japanese Unexamined Patent Publication, Tokukai, No. 2001-218046 (published on Aug. 10, 2001) discloses a method in which a similar peak is determined from a degree of similarity between a current block and a block located within a region which is distanced from the current block by a given number of pixels, and if the region is a halftone region, a halftone frequency is determined (i.e., found out) based on a peak nearest to a center of the halftone region.
Moreover, Japanese Patent No. 3093235 (issued on Oct. 3, 2000), and Japanese Unexamined Patent Publication No. 2002-77623 (published on Mar. 15, 2002) disclose a method in which halftone frequency determination is performed based on a number of peak pixels, which is a number of peak pixels in a predetermined number of block where the peak pixels are found using a mask of M pixels×N pixels (where M and N are integers predetermined experimentally).
Moreover, for example, Japanese Unexamined Patent Publication No. 2004-96535 (published on Mar. 25, 2004) discloses a method in which absolute differences in pixel value between given pixels and pixels adjacent thereto are compared with a first threshold value so as to calculate out (find out) a number of pixels (low-frequency halftone pixels) whose absolute differences in pixel value are larger than the first threshold value, and then this number of the pixels is compared with a second threshold value so as to obtain a comparison result on which the halftone frequency of a halftone region is estimated (i.e., determined).
Moreover, Japanese Unexamined Patent Publications, Tokukai, No. 2004-102551 (published on Apr. 2, 2004), and No. 2004-328292 (published on November 18) disclose methods for determining a halftone frequency based on a number of changeover (i.e., transition number) of the binary values of binary data of an input image.
According to Japanese Unexamined Patent Publication, Tokukai, No. 2001-218046 (published on Aug. 10, 2001), whether the halftone is composite color halftone or single-color halftone is not taken into consideration it is difficult to accurately determine the halftone frequency with respect to the composite color halftone region.
According to Japanese Patent No. 3093235 (issued on Oct. 3, 2000), and Japanese Unexamined Patent Publication No. 2002-77623 (published on Mar. 15, 2002), the halftone frequency determination is performed based on the number of peak pixels of the predetermined number of blocks. However, a composite color halftone and a single-color halftone of like halftone frequency give largely different numbers of peak pixels, where the composite color halftone is a halftone consisting of at least two of cyan (hereinafter, C), magenta (hereinafter, M), yellow (hereinafter, Y), and black (hereinafter K), and the single-color halftone is a halftone consisting of one of CMYK. In other words, it is difficult to distinguish the composite color halftone and the single-color halftone having similar numbers of peak pixels but different halftone frequencies. For example, it is difficult to distinguish a 133 line/inch composite color halftone and 175 line/inch single-color halftone, which have similar numbers of peak pixels. Therefore, it is impossible to extract a number of peak pixels of a particular color component.
Moreover, Japanese Unexamined Patent Publications, Tokukai, No. 2004-96535 (published on Mar. 25, 2004) extracts the low-frequency halftone pixels whose absolute differences in pixel value between the given pixels and adjacent pixels are larger than the first threshold value, and the judgment as to whether the halftone is low or high is made based on the number of the low-frequency halftone pixels. Therefore, in is difficult to determine the halftone frequency accurately.
In the methods disclosed in Japanese Unexamined Patent Publications, Tokukai, No. 2004-102551 (published on Apr. 2, 2004), and No. 2004-328292 (published on November 18), the halftone frequency is determined based the number of changeover (i.e., transition number) of the binary values of the binary data of the input image, but no information of density distribution is taken into consideration. Therefore, with this method, binarization of a halftone region in which density transition is high is associated with the following problem (here, what is meant by the term “density” is “density in color, that is, pixel value in color”. So, for example, what is meant by the term “pixel density” is “density of color of the pixel”, but not population of the pixels”).
FIG. 32(a) illustrates an example of one line along a main scanning direction of segment blocks in a halftone region in which the density transition is high. FIG. 32(b) illustrates the change of the density in FIG. 32(a). Here, it is put, for example, that a threshold value th1 illustrated in FIG. 32(b) is used as a threshold value for generation of binary data. In this case, as illustrated in FIG. 32(d), the segment blocks are discriminated into white pixel portions (that represent low-density halftone portion) and black pixel portions (that represent high-density halftone portion), thereby failing to attain such extraction in which black pixel portions (that represent a printed portion in the halftone) are extracted as illustrated in FIG. 32(c). Use of the other threshold value values th2a and th2b gives the same result. With such extraction as illustrated in FIG. 32(d), it is impossible to generate binary data that reproduce halftone frequency accurately. This results in inaccurate halftone frequency determination.